That's impossible since the JS is loaded in the page. Best way is to obfuscate the code. For example this javascript file from Google Maps. It would take a very long time to figure it out that code.

Sending the people to a 403 page when accessing the javascript will break your page IIRC. The browser sends the request for the javascript page not the server so when someone tries to load the page the request for the javascript will return a 403.

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Eric